Wireless energy transfer or wireless power is the transmission of electrical energy from a power source to an electrical load without interconnecting wires. Wireless transmission is useful in cases where interconnecting wires are inconvenient, hazardous, or impossible. With wireless power transmission, efficiency is an important parameter.
A common form of coupling in wireless power transmission is inductive coupling. A wireless power transfer system usually consists of electromagnetically coupled transmitting and receiving coils. Energy from the primary side can be transferred to the secondary side over a distance using the coil coupling. Electromagnetic induction wireless transmission techniques are near field over distances comparable to a few times the diameter of the device or devices approaching one quarter of the wavelength used.
Electromagnetic induction works on the principle of a primary coil generating a predominantly magnetic field and a secondary coil being within that field so that a current is induced in the secondary. Coupling should be tight in order to achieve high efficiency. As the distance from the primary is increased, more and more of the magnetic field misses the secondary. Even over a relatively short range the induction method is rather inefficient, wasting much of the transmitted energy.
Common uses of inductive coupling chargers are charging the batteries of portable devices such as laptop computers, cell phones, medical implants, and electric vehicles. Resonant converters may be used in both the wireless charging pad (the transmitter circuit) and the receiver module (embedded in the load) to maximize energy transfer efficiency. This approach is suitable for universal wireless charging pads for portable electronics such as mobile phones. It has been adopted as part of the Qi wireless charging standard. It is also used for powering devices having no batteries, such as RFID patches and contactless smartcards, and to couple electrical energy from the primary inductor to the helical resonator of Tesla coil wireless power transmitters.
Inductive charging is what happens when two devices—one designed to transmit power and the other designed to receive it—touch one another and energy is transferred between them. The power transmitting device projects an electromagnetic field. If the receiver is placed within that electromagnetic field, power may be transferred from the transmitter to the receiver. The receiver can be a power supply for any load. In one implementation, charging pads are able to intelligently communicate back and forth with the devices they're charging using the electronic field. In this application, the electromagnetic field used to transfer energy is modulated, allowing communication between the charging pad and the device it is charging. However, it is not always evident to the transmitting device whether there is an appropriate receiver within the electromagnetic field. Therefore, it would be useful to determine if an object were present in the electromagnetic field.